Ion binding properties in acetonitrile of cyclo‐peptides built u from proline and glycine residues

Ion binding properties of antamanide‐like cyclopeptides cyclo‐(Pro2–Glyn‐Pro2–Glym) (n, m = 1–3) have been studied by CD spectroscopy and by conductivity measurements. Cyclo‐(Pro2–Gly‐Pro2–Gly) forming complexes of different stoichiometry can be characterized by a strong preference of selectivity for Mg++ and Ca++ ions over alkali ions whereas the other members of the series bind selectively alkali and alkaline earth cations with ion radius of about 1 Å. Three main types of CD spectra of cyclic peptides and their complexes can be differentiated. Type I showing two negative Cotton effects at around 220 nm and 200 nm (Ib, Fig. 3), type II showing a positive band around 220 nm and a strong negative one below 190 nm (e.g. Ic in acetonitrile, Fig. 4), and type III showing a strong negative band in the 205 nm region (e.g. metal complexes of Id, Fig. 6)